Japanese Patent Laid-Open No. (“JP”) 08-014852 proposes a method for finding a refractive index distribution of a flat-shaped test object using two beams having different wavelengths. JP 02-008726 proposes a method for finding a refractive index distribution of a test object by measuring a transmitted wavefront while the test object is immersed in each of two types of matching oils each having a slightly different refractive index from that of the test object.
The method disclosed in JP 08-014852 requires working of the test object into the flat shape. The working of the test object is time-consuming, and changes the internal stress of the test object and thus its internal refractive index distribution before and after the working, thereby causing a refractive index distribution to be different from the pre-working refractive index distribution.
The method disclosed in JP 02-008726 requires preparing two types of matching oils having refractive indices approximately equal to the refractive index of the test object. An adjustment of the refractive index of the matching oil requires a blend of different types of oils and this adjustment is time-consuming. In addition, the matching oil having a high refractive index provides a low transmittance. Therefore, when the transmitted wavefront of the test object having a high refractive index is measured in accordance with the method of JP 02-008726, a detector can output only a weak signal, deteriorating the measuring precision of the test object.